[AMDGPU] Make GCNRegBankReassign assign based on subreg banks

When scavenging consider the sub-register of the source operand
to determine the bank of a candidate register (not just sub0).
Without this it is possible to introduce an infinite loop,
e.g. $sgpr15_sgpr16_sgpr17 can be assigned for a conflict between
$sgpr0 and SGPR_96:sub1.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D84910